| Using high-strength steels can reduce structural weight,and it has significant economic and social benefits.Meanwhile,using high-strength steels in prefabricated semi-rigid steel structural systems will improve further the material efficiency in the construction industry,which is conducive to energy saving and emission reduction for the realization of“peak carbon dioxide emissions”and“carbon neutrality”in China.High-strength steels are significantly different from conventional mild or low-alloy steels in that the former steels have higher yield-to-tensile strength ratio and lower ductility than the latter ones,which will make a difference in their beam-to-column joint behavior.However,current research on seismic performance of high-strength steel joints is insufficient,resulting in low economic benefits and even potential safety hazards when high-strength steels are applied in engineering structures.Since the structural performance is directly affected by the joint behavior,it is crucial to evaluate the seismic performance of high-strength steel beam-to-column joints.In this dissertation the seismic performance of Q690 high-strength steel extended end-plate beam-to-column joints was investigated.The main work that has been accomplished with the outcomes are as follows:1)10 Q690 high-strength steel bolted T-stub specimens were tested under cyclic loading.The hysteretic behavior of T-stubs of three groups of flange thickness was examined under both constant and variable amplitudes.The results show that,the failure modes of the T-stubs subjected to cyclic loading were always characterized with the cracking in the heat-affected zone at the intersection of the flange and web.When the T-stubs were subjected to variable-amplitude loading,the thinner the flange,the higher the cumulative plastic deformation capacity,but there existed an optimal flange thickness corresponding to the highest cumulative energy dissipation capacity.When the T-stubs were subjected to constant-amplitude loading,the number of hysteretic cycles prior to failure,cumulative plastic deformation and energy dissipation capacities were significantly affected by the flange thickness,and all those quantities decreased with the increase of the cyclic deformation amplitude.The plastic resistance of Q690 high-strength steel bolted T-stubs under tension can be still predicted by the formula in Eurocode 3,while the ultimate resistance is about 1.2 times the plastic resistance.2)Both low-cycle fatigue curves and formulae to predict the cumulative energy dissipation capacity were calibrated for Q690 high-strength steel bolted T-stubs,and a hysteretic model for the T-stubs was explored to consider the resistance and stiffness degradations.The results show that,the low-cycle fatigue failure of the T-stubs could be predicted by the formula asΔ?p=26.8Nf-0.52,whereΔ?p is the cyclic plastic deformation amplitude and Nf is number of hysteretic cycles prior to failure.The cumulative energy dissipation capacity Ecc for a given plastic deformation amplitude?p could be predicted by the formula as Ecc/E0=0.036（tf?p/2m2）-1.403,where E0is the energy consumption under monotonic tension up to?p,tfis the T-stub flange thickness and m is the distance from the bolt to the flange and web interface.The current hysteretic model developed for conventional steel bolted T-stubs could hardly predict the hysteretic response of Q690 high-strength steel bolted T-stubs.3)5 Q690 high-strength steel beam-to-column joint specimens were tested under cyclic loading.The hysteretic behavior of joints of three end-plate thickness,two column web panel thickness and two column flange thickness were investigated.The results show that,the failure modes of the joints were quite similar to those of traditional ones made of conventional steel.The high-strength steel extended end-plate had certain plastic deformation and energy dissipation capacities.Its maximum gap rotation reached 0.02～0.03 rad,while its cumulative plastic rotation reached 0.1～0.2 rad.Moreover,there existed an optimal end-plate thickness corresponding to its highest cumulative energy dissipation capacity.The high-strength steel panel zone had stable and excellent plastic deformation and energy dissipation capacities.Its maximum shear distortion capacity should be above 0.04 rad,while its cumulative plastic deformation capacity should be above 0.8 rad.The high-strength steel column flange,when designed to be thinner than the end-plate,could dissipate energy by yielding.However,this might have some adverse effect for the joint seismic performance by causing premature bolt failure.The plastic flexural resistance of Q690 high-strength steel extended end-plate joint can be still predicted by the component method in Eurocode 3,while the ultimate flexural resistance is 1.1～1.2 times the plastic flexural resistance.4)Numerical simulation was carried out on the hysteretic behavior of Q690 high-strength steel extended end-plate beam-to-column joints.The comparison between the simulation and the experiment shows that,the three-dimensional finite-element analysis provided results that were well correlated with the experimental ones.The analysis predicted accurately the pinching behavior in the hysteretic loops of bolted end-plate connections and their gap deformation,as well as the plump hysteretic loops of panel zones and their shear distortion mode.5)Current seismic design methods in the Chinese,American and European codes were compared for bolted end-plate beam-to-column joints.Several seismic design recommendations were proposed with respect to Q690 high-strength steel joints are:a)when designed to dissipate energy,the limit on the high-strength steel end-plate thickness should be increased as 1.2～1.4 times the current one in Eurocode 8;b)the high-strength steel column flange should not be thinner than the end-plate,if continuity plates are used;c)the high-strength steel column web panel shear deformation is allowed to contribute no more than 70%of the joint plastic rotation capability,but meanwhile the influence of the panel zone deformation should be considered in seismic analysis of the structure. |
